There is a need for improved actuation for medical devices. One important example is the surgical snare handle. Snares are used to surgically remove polyps (pre-cancerous tissue growth in the colon and rectum), to remove esophageal carcinoma, and to perform nephrostomy. A snare device is inserted through a working channel of an endoscope fitted with a fiber optic camera, the output of which is attached to a video monitor.
In the case of a polyp, a doctor manipulates the endoscope in the tract of the patient until a polyp is located. The end of the snare in the form of a wire loop is extended and positioned around the polyp, and at the doctor's request, an assistant, typically a nurse, retracts the snare to cause the loop to retract and resect the polyp. An electrical current, such as r-f, is passed through the loop to cauterize the polyp and prevent bleeding.
The same technique is employed for other physical features to be excised or cut by a snare.
The snare is controlled by a snare handle. It is desirable that a snare handle have great fidelity so that whatever resistance is experienced by the snare is felt through the handle by the operator. If the operator cannot feel what is happening at the snare end, the polyp or other tissue might be severed before the cautery current has been applied, causing bleeding and other harmful consequences.
It is desirable that the snare be capable of extending fully and of retracting into the sheath a certain distance as a safety margin, for instance 0.75 inches. It is also desirable that the handle be constructed so that nurses with small hands can operate it.
Handles having good tactile feeling have long been available. These handles typically have one-to-one activation ratio in which the actuating element of the handle moves the same amount as the snare. Because the distance the snare must move is fairly large, these handles have been difficult to operate with one hand, especially for people with small hands. Attempts have been made to provide a handle which moves only a fraction of the distance of the snare travel. The presence of intervening parts, which may be subject to deformation under load, between the snare loop and the actuating member, however, has rendered the tactile sensitivity of these handles sub-optimal. Furthermore, these handles have also been complex, expensive to manufacture and have had other disadvantages.